A novel sperm adaptation to evolutionary constraints on reproduction: Pre‐ejaculatory sperm activation in the beach spawning capelin (Osmeridae)

Abstract

Reproduction of external fertilizing vertebrates is typically constrained to either fresh or salt water, not both. For all studied amphibians and fishes, this constraint includes immotile sperm that are activated after ejaculation only by the specific chemistry of the fertilizing medium in which the species evolved (fresh, brackish, or salt water). No amphibians can reproduce in the sea. Although diadromous fishes may migrate between salt and fresh water, they are shackled to their natal environment for spawning in part because of sperm activation. Here, we report for the first time among all documented external fertilizing vertebrates, that in the absence of any external media, sperm are motile at ejaculation in a marine spawning fish (Osmeridae, capelin, Mallotus villosus). To illuminate why, we evaluated sperm behavior at different salinities in M. villosus as well as the related freshwater spawning anadromous rainbow smelt (Osmerus mordax). Surprisingly, sperm performance was superior in fresh water for both species. M. villosus spend their entire life at sea but our results show that their sperm are deactivated by sea water, suggesting a freshwater ancestry. By circumventing constraining water chemistry, we interpret the unique pre‐ejaculatory sperm activation in this species as a novel adaptation that enables fertilization in the marine environment. These findings also contribute to understanding the persistence of anadromy, despite great energetic costs to adult fishes.

Assessing the potential for post‐ejaculatory female choice in a polyandrous beach‐spawning fish

“…Both pre‐ and post‐ejaculatory sexual selections are well established in the closely related Salmonidae, but there are key differences in gametes of the smelt family (Osmeridae) which might limit the possibility of cryptic female choice. For example, capelin have structurally different sperm than salmonids (Beirão, Lewis, & Purchase, ), and unlike salmonids (10%–30% total egg volume; Lahnsteiner et al, ), there is very little ovarian fluid (~1% of total egg volume) released with capelin eggs (Beirão, Lewis, et al, ). To be thorough, our experiment was designed to evaluate the potential influences of both ovarian fluid and any chemicals released from eggs (Morisawa, Tanimoto, & Ohtake, ), including the adhesive egg coat (Burnett et al, ; Tholl et al, ).…”

“…In many externally fertilizing fishes, ovarian fluid (i.e., coelomic or peritoneal fluid) is released with eggs (Zadmajid et al, ) and can mediate sperm swimming characteristics and potentially facilitate cryptic female choice (e.g., Alonzo et al, ; Elofsson, Van Look, Sundell, Sundh, & Borg, ; Lehnert, Butts, et al, ; Lehnert, Devlin, Heath, & Pitcher, ; Poli, Immler, & Gasparini, ; Rosengrave, Gemmell, Metcalf, McBride, & Montgomerie, ; Urbach, Folstad, & Rudolfsen, ). Capelin have low quantities of ovarian fluid (~1% of total egg volume; Beirão, Lewis, et al, ) compared to closely related salmonids (10%–30% total egg volume; Lahnsteiner, Weismann, & Patzner, ), which might limit opportunities for post‐ejaculatory sexual selection. However, capelin eggs instantly stick to substrate (Templeman, ) and it is possible the adhesive mechanism affects sperm, as it does in anurans (Burnett, Sugiyama, Bieber, & Chandler, ; Tholl et al, ).…”

“…Videos were analysed with a computer‐assisted sperm analysis (CASA) plugin for ImageJ (Wilson‐Leedy & Ingermann, , modified by Purchase & Earle, ) and sampled in half‐second intervals. CASA plugin input parameters were modified from Beirão, Lewis, et al () so the software could differentiate motile and drifting sperm (Table S3). Measurements of sperm characteristics were based on an average of 141 + 79 SD sperm tracks per unique video.…”

“…Models were fit with restricted maximum likelihood (REML) using the ‘lme4’ package (Bates, Maechler, Bolker, & Walker, ) in R 3.4.3 (R Core Team, ). Time elapsed since the ejaculate was mixed with the test solution was included as a covariate because capelin sperm swimming declines rapidly with time (Beirão, Lewis, et al, ). The significance of fixed effects was assessed using the ‘lmerTest’ package (Kuznetsova, Brockhoff, & Christensen, ) and the Satterthwaite approximation for degrees of freedom.…”

“…Shaking of eggs does not break them up as they are adapted to incubate on the beach with high surf. We observed sperm swimming in different test solutions made using 15 psu degassed tap water and 0.1% BSA (Beirão, Lewis, et al, 2018;Beirão, Litt, & Purchase, 2018). We chose to set treatment water at 15 psu because sperm swimming ability is greatly reduced at higher salinities in this species (Beirão, Lewis, et al, 2018).…”